Remote rendering over internet (separate networks)
There is a nice trick to setup a local renderfarm without any external software:


  I found this to be working very reliably for me: Make your blend file ready for rendering, with all the settings, with the whole frame range you want to render. Make sure that in the Render Output settings, Overwrite is ticked off and Placeholders is turned on. Your blend file should be on some shared folder, where all machines can access it and can write rendered frame to the same output folder. Then, you can open this same file on each machine and run "Render Animation" command. The Overwrite/Placeholders combo is the key here. Before starting to render a frame, Blender checks if it is rendered already. If it exists, blender checks for next frame. Upon starting a render, Blender creates placeholder file immediately, so it prevents different computers from rendering same frame. Over a local network, this works perfect.


Source: https://blender.community/c/rightclickselect/H3dbbc/